'I want a pig that's had a happy life," a gourmet friend of mine declared to a local butcher recently. Like him, you are probably concerned about your family's health and aware of your food shopping's impact on the environment. You dip into farmers' markets and buy organic. You are suspicious of GM crops and worried about food miles.

Peter Singer has a term to describe you: conscientious omnivore. With his new book, Eating: What We Eat and Why it Matters, the Australian philosopher also brings a challenge. As a conscientious omnivore you are a little like McDonald's: you have improved your food habits a bit but you could do an awful lot more.

Singer, professor of bioethics at Princeton University in the US and author of Animal Liberation, the seminal treatise that sparked the animal rights movement more than 30 years ago, has written, with co-author Jim Mason, not about healthy eating but the ethics of eating. The pair examine how the purchases made by three American families (a price-conscious family of supermarket shoppers, a middle-class family of conscientious omnivores and a family of vegans) affect the people, animals and the environment from which the food came.

Beginning as a familiar - if powerful - critique of industrial farming and the "fallacy" of cheap food, Singer and Mason provide unusually forthright guidance for consumers struggling with the ethical minefield that is the modern food shopping trip.

Confusing labels, disingenuous producers, conflicting research and the lack of any kind of system to weigh up the merits of, say, food miles versus organic: the time-poor consumer can easily be overwhelmed by the effort of right-thinking. Singer, who became vegetarian after meeting a graduate who refused spaghetti bolognaise in an Oxford University canteen 36 years ago, says the first ethical step meat-eaters should take is to buy organic. With US consumers spending 6% of their income on groceries compared with 17% 50 years ago, everyone should pay more for their food, he says.

"Food is absurdly cheap by all historical standards," he says. "There has never been an era in human history when people have been able to feed themselves for so few hours of work as we can today in the developed world. That cheapness has come at a price. It has imposed costs on animals, on the environment and on workers in the food industry."

Eating is not a rant against big corporations. Singer's arguments are a challenge to knee-jerk antiglobalisation campaigners for whom McDonald's is an unmitigated evil. Trapped in a small town and forced to choose an independent takeaway or the golden arches, Singer would plump for the latter (as his book points out, in the US, McDonald's has insisted its eggs come from hens given more space than the legal minimum, among other "ethical" innovations). "The fact that a big chain has a national and international reputation to protect means they need to be a bit more cautious about what they are doing than someone who has no brand and is not going to suffer from any kind of disclosure," he says.

In this country, Singer dishes out praise for Marks & Spencer and Waitrose for banning battery eggs from their shelves. Big corporations are not intrinsically unethical. "I see big corporations as following what consumers will buy. If you have sufficiently educated consumers, you can get ethical food from big corporations."

Singer makes a compelling argument against cheap food. But what about time? Trapped in market capitalism, don't we simply lack the hours to source our food ethically? He thinks we can choose to spend more time on food shopping and says the growth of farmers' markets shows that some people are treating it as "a recreational activity".

While recent reports have exposed fraudulent mislabelling of meats and other produce on farmers' markets, Singer is a big fan and sees them as a chance to chat to producers and even perform your own farm inspection. "You can say, 'Can I come and see your farm?' and if they say no you should be suspicious. If they say yes, that's a good sign but you should try and take them up on it." He accepts, however, that consumers cannot do it all themselves and must be helped by a tough regulatory framework.

He offers some sharp thinking on issues that have traditionally bam-boozled conscientious omnivores. Genetically modified crops are not dismissed as evil foods. "I don't have a general ideological objection to GM. I don't think there is anything intrinsically wrong with altering the genetic nature of beans," he says, accepting there are hazards but arguing it is not always best to apply the precautionary principle. "The difficulty here is that some of the benefits that advocates of GM talk about are significant. They may benefit developing countries. And they may lead to ways of reducing pesticide use."

On the fashionable preoccupation with food miles, Singer also offers some clear - and sometimes surprising - guidance. He argues that it is better at times to support agriculture in developing countries than "selfishly" protect local farmers. "Not all miles are the same. Miles by ship are not nearly as bad as miles by truck, and miles by truck are not nearly as bad as miles by plane. Something to be said in favour of supermarkets and centralised distribution systems is you can make one trip to the supermarket and buy your stuff there. If you are going to drive around the countryside in your four-wheel drive to pick up a dozen free-range eggs here and then pick up a lettuce there, you have probably run up more miles than a truck would."

Is it better to support developing economies over the local economy? "I don't see why we should favour British farmers over Kenyan farmers just because they are British. We should favour farmers who most need economic support to make a living, and that's likely to be developing-world countries."

More alfalfa male than alpha male, Singer makes clear that the red-blooded west's hunger for meat cannot be replicated globally. A carnivorous lifestyle is unsustainable: there isn't enough land to farm meat for all. One moral rule he suggests is to eat meat only from a farm you have visited. Most people would therefore conclude it is simpler to avoid meat. Is Singer arguing that, ultimately, veganism is the only ethically defensible position? "I wouldn't phrase it in such absolute terms. It's pretty difficult to be a conscientious omnivore and avoid all the ethical problems, but if you really were thorough-going in eating only animals that had had good lives, that could be a defensible ethical position. It's not my position, but I wouldn't be critical of someone who was that conscientious about it."

When he is visiting London, as he is now, Singer enjoys dining at the (modest) Soho vegetarian restaurant Govindas. Is dining in a fancy restaurant ever ethically OK? "I don't think there is something objectionable as such to eating out. It is good to talk to the people in the restaurants about where their food comes from and let them know their customers are interested in whether it is organically produced."

Do people eat out too much? "Yes. A lot of fine dining is really a kind of gluttony or status-seeking. When people go out for a meal and spend something that could have fed a family in Africa for three months that strikes me as something that is not defensible."

So is eating for pleasure ever defensible? Singer smiles. "I'm not that much of a puritan. Gee, I think pleasure is a good thing. I just wish that everybody, animals included, could experience more pleasure in their lives and less suffering"

· Eating: what we eat and why it matters by Peter Singer and Jim Mason is published by Arrow, £7.99.